To remain competitive, electrical utility companies continually strive to improve system operation and reliability while reducing costs. To meet these challenges, the utility companies are developing techniques for increasing the life of installed equipment, as well as diagnosing and monitoring their utility networks. Developing these techniques is becoming increasingly important as the size and demands made on the utility power grid continue to increase. A utility power grid is generally considered to include both transmission line and distribution line networks for carrying voltages greater than and less than about 25 kV, respectively.
Voltage instability on the utility power grid is a critical problem for the utility industry. In particular, when a fault occurs on the transmission grid, momentary voltage depressions are experienced, which may result in voltage collapse or voltage instability on the grid.
Various equipment and device solutions have been developed to address voltage instability problems. The term “Flexible AC Transmission Systems” (FACTS) is used to describe technologies to enhance the capacity and stability of power transmission systems. These systems operate by temporarily injecting power into the system.
One FACTS technology is dynamic shunt compensation in which a dynamic shunt compensator connected in parallel with the power system automatically and instantaneously adjusts its reactive power output by injecting real and/or reactive power into the system in response to line voltage disturbances. A dynamic shunt compensator may be referred to more generally as a type of reactive power compensation system. FACTS devices may also be series-connected to the power system.
Reactive volt-amperes are expressed in VARs; a term coined from the first letters of the words “volt amperes reactive.” Reactive volt-amperes considered over a period of time represent oscillations of energy between the source and the load. Transmission systems require reactive power as part of their fundamental operation. The reactive power sets up magnetic fields in the transmission cables and transformers that allow “real” power to flow. Generating or absorbing reactive power at a given point on a transmission system is the primary means of regulating the voltage at that point. In particular, if it is determined that a line voltage is too high, then an inductive current is injected into the line (i.e., reactive power absorption) to lower the line voltage; whereas, if the line voltage is too low, then a capacitive current is injected (i.e., reactive power generation) to raise the line voltage.
One type of dynamic shunt compensator, called a Static VAR Compensator (SVC), generates reactive power from a bank or switched banks of capacitors. A thyristor-switched inductor is connected in parallel with the capacitors to partially or fully absorb the VARs generated by the capacitors. As the conduction phase angle of the thyristor switch is varied from full on to off, lesser amounts of VARs will be absorbed and the SVC's net VAR output becomes variable and hence capable of adjusting the voltage on the network. The SVC continuously shifts the phase angle (VAR output) in response to dynamic power swings on the transmission network due to changing system conditions.
Another type of dynamic shunt compensator, called a Static Synchronous Compensator (STATCOM), uses power electronics (e.g., a voltage sourced inverter) to generate the VARs. Like an SVC, a STATCOM generally includes one or more step-up transformers to convert the reactive power to the appropriate voltage level for coupling to the transmission system. The power electronics generally includes an inverter whose output current phase is controlled to lead the output voltage by 90 electrical degrees when generating (capacitive) VARs or to lag the voltage by 90 degrees when absorbing (inductive) VARs.
An example of a STATCOM system is the D-VAR® system manufactured by American Superconductor Corporation of Westboro, Mass. described in U.S. Pat. No. 6,987,331 entitled Voltage Recovery Device for use with a Utility Power Network. The D-VAR® system can be configured to provide up to hundreds of megaVARs (MVARs) of reactive compensation. The amount of reactive power delivered per unit is typically on the order of 1 to 8 MVARs continuous, with an instantaneous reactive power output up to approximately 24 MVARs per unit. A modular version of the D-VAR® system is comprised of modular units generally referred to as power electronics enclosures, such as the PowerModule™ enclosure (PME) manufactured by American Superconductor Corporation of Westboro, Mass. The PME resembles metal enclosured switchgear with approximate dimensions of 8 feet by 8 feet by 8 feet.
Various configurations of reactive power compensation systems are possible. For example, a Dynamic VAR Compensator (DVC™) system manufactured by American Superconductor Corporation of Westboro, Mass. is a configuration that employs switched capacitors and/or inductors in combination with a D-VAR® system, to augment the overall reactive power rating. The DVC™ system is described in published U.S. patent application Ser. No. 10/794,398.
Another reactive power compensation system configuration uses a series impedance upstream of the D-VAR® system and is capable of restoring voltage sags on the transmission system to within acceptable limits at a load. Such systems are used in applications where a substation feeds a dedicated load at which power quality is paramount (e.g. semiconductor fabrication facility). An example of such a system is the Power Quality-Industrial Voltage Restorer (PQ-IVR™) system manufactured by American Superconductor Corporation of Westboro, Mass. and described in U.S. Pat. No. 6,392,856 entitled Method and System for Providing Voltage Support to a Load Connected to a Utility Power Network.
Still another type of reactive power compensation system configuration is a Distributed Superconducting Magnetic Energy (D-SMES) storage system, which refers to a STATCOM having energy storage capability. One such system is described in a U.S. Pat. No. 6,906,434 entitled Electric Utility System with Superconducting Magnetic Energy Storage.
Traditional STATCOMs have large, fixed ratings on the order of 25 MVA to 100 MVA and are customized for each customer/application. Also, per the ANSI standard, utility substation transformers designed for natural convection cooling are rated for 30° C. average ambient temperature over any twenty-four hour period and 40° C. maximum. These factors make it difficult to provide modular, scaleable reactive power compensation systems based on standard “building blocks,” such as standard transformers. For example, it is not efficient to use the same transformer for D-VAR® systems installed in climates warmer than the ANSI standard, since such installations would require a higher rated transformer or one that is fan cooled, whereas no such requirement is necessary in cooler climates.
One way to provide a “standard” transformer solution with an ANSI standard transformer is to use supplemental fan cooling for the transformer in warmer climates. Alternatively or additionally, in such warmer climates, the ANSI standard transformer may be operated at less than its full power specifications (i.e., derated). However, both of these approaches add cost.